Hydraulic systems are used to supply pressurized hydraulic fluid to one or more fluid actuators in many types of machines including vehicles such as construction vehicles (e.g., loader/backhoes, skid-steers, forklifts, excavators, etc.), agricultural vehicles (e.g., tractors, combines, etc.) and other types of vehicles for performing work (e.g., over-the-road trucks, garbage trucks, etc.). Such hydraulic systems are also used to supply pressurized hydraulic fluid in stationary machines. For clarity, it is assumed below that the machine is a loader/backhoe construction vehicle similar to the 590 Super L model loader/backhoe vehicle made by Case Corp. of Wisconsin. The hydraulic system of the 590 Super L loader/backhoe currently uses two fixed displacement pumps having a displacement of over 5 cubic inches. However, the hydraulic system described herein may be used in other machines.
Existing machine hydraulic systems are typically produced in either of two forms: open center systems which use one or two fixed displacement pumps (typically gear pumps or vane pumps) that deliver flow in proportion to their speed (i.e., rpm) on a continuous basis; and closed center systems which use one or two variable displacement pumps to produce a variable flow on demand. The following paragraphs describe both types of systems, with the assumption that the machine is being operating at rated speed as recommended by most equipment manufacturers. According to industry custom, fixed displacement pumps are referred to below by the symbol PF and variable displacement pumps are referred to by the symbol PV.
The PF pump or pumps (e.g., gear pumps) used by open center or continuous flow hydraulic systems have the advantageous characteristics of being low cost and highly responsive. However, PF pumps are typically unreliable at high pressures and are inefficient at particular operating conditions such as during metering or at tool stall. For example, assume that the operator of a loader/backhoe vehicle equipped with a hydraulic system with a PF pump is attempting to precisely position the backhoe and is, therefore, using only a portion of the flow being output by the PF pump to move the given backhoe cylinder. The PF pump is consuming power equal to its total output flow and pressure required, even though the backhoe is using only a portion of that flow. The unused flow is converted to heat, and fuel is being consumed unnecessarily. The extreme situation occurs when the backhoe is stalled and the total flow is going over a relief valve and is, therefore, not doing any useful work. In this situation, the total flow from the PF pump is merely generating heat, and large volumes of fuel are being consumed with no work being performed. To reduce this wasteful pump operation, some hydraulic systems use multiple PF pumps called upon to deliver a required fluid volume or pressure depending on the operating or load condition. This solution, however, cannot always provide the correct fluid volume, and the PF pumps are still unreliable at high pressures.
The PV pump or pumps (e.g., piston pumps) used by closed center machine hydraulic systems produce a variable flow on demand. Thus, in standby conditions, such systems do not circulate hydraulic fluid. When such systems are equipped with flow and pressure control, the operator has the ability to direct the system to provide only a small volume of fluid to the work circuit which actuates the tool (e.g., the backhoe or loader), and the PV pump produces only the volume needed. When the tool stalls, the PV pump reduces its output flow to near zero, with corresponding reductions in the amounts of heat generated and fuel consumed.
Although hydraulic systems using only PV pumps have advantages in comparison to systems using only PF pumps, as described above, such systems also suffer from disadvantages as described below. A first problem of hydraulic systems using only PV pumps is the slow response of such systems. For example, assume the operator of a loader/backhoe vehicle wants to move or accelerate an attachment (e.g., bucket) quickly with the PV pump in stand-by condition (i.e., the de-stroked condition with no fluid being pumped). Since the PV pump is in stand-by when an instantaneous demand for fluid occurs, a finite time period is required for the pump to reach its full stroke where it will start pumping a large fluid volume. This finite period will result in hesitation (i.e., slow acceleration) of the tool, which is typically noticeable by the operator who expects and desires immediate tool movement. This situation occurs, in a more specific example, when a backhoe operator tries to shake out mud stuck in a bucket. In existing closed center systems, the slow response of PV pumps do not provide the instantaneous response needed to shake out the mud, and only a "mushy" shake will occur which may be insufficient to knock the mud out. In contrast, in conventional open-center hydraulic system, a control valve will be slammed open and closed to instantaneously start and stop fluid flow to the work circuit and a "hard" shake will occur which will be sufficient to knock the mud out.
A second problem of hydraulic systems using only PV pumps is the high cost of PV pumps in comparison to PF pumps of similar displacement volume. The cost of PV pumps is high due to the higher complexity and more moving parts required for PV pumps compared to PF pumps. In addition, the cost of the larger PV pumps required to accommodate applications requiring high displacements, such as most construction vehicle applications, increases disproportionately with size due to the relatively low volumes in which the larger displacement pumps are made and sold. Thus, to obtain an economical system, the hydraulic system must use the PV pumps sold in high volumes which are traditionally the smaller displacement pumps not suitable for the large-size applications such as those for construction vehicles.
A third problem of hydraulic systems using only PV pumps is the difficulty in obtaining a PV pump in the size needed for a particular application. In other words, since manufacturers do not make PV pumps having a wide variety of displacements, it can be difficult to obtain PV pumps with displacements customized to the particular application. Thus, for example, if a particular application requires a PV pump having a displacement of 5 cubic inches, it may be necessary to use a 6 cubic inch pump and then limit its stroke to 5 cubic inches. Custom-sizing does not pose a significant problem for PF pumps such as gear pumps since it is relatively easy for the manufacturer to shave the gear to obtain the desired displacement.
A fourth problem of hydraulic systems using only PV pumps involves durability issues which can arise when such systems are used for operating reciprocating devices such as hammers. When running a reciprocating tool, each operating cycle starts with a pressure spike and then a pressure drop, with relatively high fluid flow. If the pressure drops to zero, durability problems can occur due to problems associated with keeping the slippers located within the PV pumps in place.
A fifth problem of hydraulic systems using only PV pumps is the inability to filter or cool the system's hydraulic fluid under all operating conditions. Thus, for example, when the system is in stand-by or at stall, there is no fluid flow. With no flow, no fluid will pass through the system's filter and cooler components, and no filtering or cooling will occur. The lack of filtering and cooling will cause the hydraulic system to run hotter and dirtier, and can lead to reliability problems.
To solve some of the problems associated with pure PV or pure PF pump hydraulic systems, previous systems have combined PV and PF pumps along with a modulating unloading valve to unload the PF pump. Such prior hybrid dual pump systems, however, have been subject to problems such as jerky operation, the need for complex modulated unloading valves, and inefficiencies due to modulation. For example, due to the fast drop-off in flow after pressures reach a predetermined value which is typical of the output characteristics of PV pumps, such prior systems need to use a modulating valve for unloading the gear pump. However, from an efficiency viewpoint, it would be better to cut the flow from the gear pump in and out of the flow applied to the work circuit quickly. By modulating the pressure (i.e., bringing on or taking off the flow from the gear pump slowly), such previous systems waste horsepower since only part of the output flow from the gear pump is used, and the rest of the flow from the gear pump is wasted as heat. Thus, due to the need for modulation, such prior dual pump systems are inefficient and wasteful.
Thus, it would be advantageous to provide an improved variable flow hydraulic system including a PF pump integrated with a PV pump to provide for the performance and efficiency advantages of pure PV pump systems, while minimizing costs by using a PV pump of a size produced in high quantities with a low-cost PF pump. It would also be advantageous to provide such a hybrid dual pump hydraulic system wherein the output flows of the PF pump and the PV pump are combined using the inherent flow compensation characteristics of the PV pump to provide for smooth, accurate, responsive and efficient machine operation. Further, it would be advantageous to provide such a hybrid dual pump system which eliminates some or all of the above-described disadvantages of hydraulic systems using only PV pumps.